Method for determination of copper-chrome-arsenic preservatives in treated wood



United States Patent US. Cl. 23230 3 Claims ABSTRACT OF THE DISCLOSURE Content of copper-chrome-arsenic preservative in treated wood is determined by dissolving the chromium from the wood using a predetermined amount of a solution of hypochlorous acid or sodium hypochlorite. By measuring the color intensity of the resulting solution, the amount of preservative in the wood is ascertained.

In recent years much progress has been made in the field of preservatively treating wood with water borne salts. Beginning 'with the soaking of timbers in brines from the sea, water soluble salts have been used since earliest times for treating wood.

The use of water soluble salts has continued in spite of the fact that the chemicals used were leached from the wood when it was exposed to water in service. When creosote, which is an oily material not easily leachabl-e by water, was introduced as a wood preservative about 150 years ago, it failed to replace the water soluble salts completely in spite of their major weakness of water leachibility because for many applications like house framing, siding, etc. creosoted wood was undesirable because of its oiliness, dark color, smelliness and unpaintability.

After some years of experimenting, wood preserving chemists found, about 30 years ago, a series of water soluble salt formulations that because of chemical reactions with the wood, became insoluble and were fixed to the wood in such a way that they were virtually nonleachable. The formulations are mixtures of compounds or complexes of copper, chromium, and arsenic, often containing other constituents. Exemplary of such preervatives are chromated copper arsenate and copperized chromated zinc arsenate. Workers in the art refer to these formulations as copper-chrome-arsenic preservatives and that term will be used herein. Under various trade names such as Celcure, Greensalt, Boliden Salt, Tanalith C. etc., copper-chrome-arsenic preservatives are used throughout the world, competing on an even basis of performance with creosote and the oil-borne preservatives, pentachlorophenol and copper naphthenate.

Copper-chrome-arsenic preservatives are so fixed in the wood that heretofore it has been impossible to extract them from sample borings taken from the wood for the purpose of determining how much preservative the wood has retained; a quality control procedure used to inspect treated timber to see whether it contains enough preservative to assure its lasting in the service for which it is to be used.

Creosote treated lengths of wood are inspected by taking borings from the treated wood and extracting the creosote with a solvent such as toluene or dimethylformamide but no solvents satisfactory for quantitative extraction of the copper-chrome-arsenic complexes formed in the treated wood have been found so far. Because of this, in order to determine the content of such salts in wood, it has been necessary to carry out a wet digestion to completely destroy the organic portion of the wood leaving the inorganic copper-chrome-arsenic constituents 3,528,716 Patented Sept. 15, 1970 in a form where they could be determined by conventional methods of analysis. This procedure is described in the American Wood Preservers Association Standard A7-67: Standard Wet Ashing Procedure for Preparing Wood for Chemical Analysis. However, this standard procedure uses the very dangerous oxidant perchloric acid. Because there have been numerous violent explosions with this reagent, the wet digestion standard, and the related analytical methods, carry frequent and emphatic warnings of the danger of explosion.

Accordingly, an object of this invention is to provide a method for determining the content of copper-chromearsenic preservative in treated wood without the disadvantages inherent in currently available techniques.

Another object is to provide such a method which employs commonly available and safe reagents.

In obtaining the objects of this invention, one feature resides in contacting a predetermined volume of Wood treated with copper-chrome-arsenic preservative with a predetermined volume of an aqueous solution of hypochlorous acid or sodium hypochlorite to thereby leach chromium ion from the wood. The color intensity of the resulting solution is then measured to ascertain the amount of preservative in the wood.

Other objects, features and advantages of the invention will be apparent to those skilled in the art from the description of the invention which follows.

I have found that components of the copper-chromearsenic preservative can be dissolved from treated wood by using solutions of hypochlorous acid, or sodium hypochlorite. If the solution has a pH below about 7, it will attack the wood producing colored extracts that interfere with the colorimetric determination. By adjusting the pH of the solution to above 7, preferably in the range 10 to 12, the wood is not attacked and the only color present is that produced by the chromium ion. The copper is left on the wood. The arsenic also is dissolved from the wood but this ion does not impart color to the solution and does not interfere with the colorimetric determination.

The amount of preservative in the wood is determined by measuring the color intensity of resulting solvent. The intensity of the color of the solvent is measured by any number of means and the amount of preservative present in the wood is quickly determined. The whole procedure is rapidly conducted in the field by the operator, and the process is performed at the prevailing atmospheric temperature and pressure.

Examples of means for measuring color intensity include the placing of the solvent whose color intensity is to 'be measured into a colorimeter having a known standard therein. From the difference in color intensities, the amount of preservative present in a cubic foot of wood is quickly determined. Also, the color of the solvent may be measured against a plurality of standard colors on a chart, each color designating a particular weight per 'volume of wood, so that the color on the chart closest in intensity to the color of the solvent indicates quickly and accurately to the operator the preservative content of the wood being tested.

Still another means which may be utilized includes a color standard consisting of a plurality of test tubes, each containing a similar solvent with an increased concentration of chromium ion therein, the concentration of the chromium ion and the relationship of chromium ion concentration to concentration of preservative in the wood in terms of weight per unit volume being known. By measuring the color intensity of the solvent being tested with the colored solvents of the standard, the amount of preservative in the wood quickly becomes known.

Also, the percent transmission of the solution can be measured with a spectrophotometer and related to a standard chart or graph comparing percent transmission with preservative content of the wood.

Various Well known techniques can be employed to prepare the sample of wood to be contacted with the solution of hypochlorous acid or sodium hypochlorite. Preferably, the procedures and apparatus disclosed in my U.S. Pats. Nos. 2,992,902 and 3,170,647 are used.

Aqueous solutions suitable for use in this invention are readily available, some of them corresponding to sodium hypochlorite laundry bleaches, sold for household use. However, freshly prepared solutions have the highest activity in this invention.

One method of preparing a solution for use in the invention is to treat an aqueous solution containing 2.5 to 10 gms./ 100 m1. NaOH with chlorine gas until the pH reaches 10 to 12. This solution is then used to extract the preservative from the wood to be tested.

The following example is merely illustrative of one embodiment of this invention and is not to be construed as limiting in any respect thereto.

An aqueous sodium hydroxide solution containing 5 gms./ 100 ml. was saturated with chlorine gas. The pH of the resulting solution was 11.5. Increment borings were taken from a charge of poles treated with a copperchrome-arsenic solution. Individual borings were ground in the grinder described in my U.S. Pats. 2,992,902 and in 3,170,647 issued to J. C. Loftin and assigned to me. The ground boring was treated with the sodium hypochlorite solution using ml. of the solution to one inch of boring approximately 0.2 inch in diameter. The resulting solution Was filtered to remove wood particles and the clear yellow solution was measured with a spectrophotometer at a wave-length of 450 The light transmission of the 4 sample read against the plain sodium hypochlorite solution as From a curve prepared from known amounts of the preservative salt, the 35% transmission was found to correspond to 1.35 lbs/cu. ft. of copperchrome-arsenic salt in the boring.

If it is desired to check the arsenic in the solution dissolved from the wood this can be done by forming the Molybdenum blue complex. If copper is to be determined, this can be extracted from the residue on the filter with a suitable solvent and determined colorimetrically with the usual copper specific reagents. However, measuring chromium ion concentration gives an accurate indication of the preservative content of the Wood.

What is claimed is:

1. In the method for determining the preservative content of wood by extracting preservative from a predetermined volume of wood with a predetermined volume of solvent and measuring the color intensity of the resulting solution to thereby ascertain the preservative content of the Wood, the improvement for determining the content of copper-chronie-arsenic preservative in Wood comprising using as the solvent an aqueous solution of hypochlorous acid or sodium hypochlorite.

2. The method of claim 1 wherein said solvent has a pH above 7.

3. The method of claim 1 wherein said solvent has a pH of 10 to 12.

References Cited Tshida et al., Chem. Abstr. 64, 6877f (1966) Mathur, V.N.P., Chem. Abstr. 64, 9933f (1966).

MORRIS O. WOLK, Primary Examiner R. M. REESE, Assistant Examiner 

